With the continuous development of communication technology, services such as data, voices and images can be transmitted through optical fibers. Table 1 shows the major services with transfer speeds at present.
TABLE 1Transfer SpeedService NameMbpsFast Ethernet FE125Gigabits Ethernet GE1250Fiber Distributed Data Interface FDDI125Store Network ESCON200Store Network Fibre Channel/FICON1062.5Digital Video Broadcasting DVB270High-definition Digital TV HDTV1485Asynchronous Transfer Mode ATM155/622Synchronous Digital Transfer STM-1/OC-3155Synchronous Digital Transfer STM-4/OC-12622
If one optical fiber is used to transmit only one service, optical fiber resource will be severely wasted. Currently DWDM system is usually adopted to transmit services, in order to greatly save optical resource. FIG. 1 illustrates the transfer principle of DWDM system in terms of the prior art. Signals are received from the service side by an Optical Transpose Unit (OTU) used for transmitting at the transmitting end. Signal optical carriers with different wavelengths are combined by a wave combining unit and then sent to an optical fiber for transmission. At the receiving end, a wave dividing unit separates the optical carriers, which hold different wavelengths and carry different signals. The OTU used for receiving transmits the received signals to each branch. Accordingly, multiplexing transfer of multi-path optical signals through one optical fiber can be implemented.
Usually there are two transfer modes when using DWDM system to transmit service data:
One transfer mode is to perform transparent transmission for services with arbitrary speed wavelength. After services with nonstandard wavelength are converted to services with standard wavelength matching DWDM system through optical/electrical/optical (O/E/O) manner, transfer of various services is implemented by 3R (Re-sharping, Re-amplifying and Re-timing) technology.
FIG. 2 is a schematic diagram of OTU for implementing transparent transmission for services with arbitrary speed wavelength according to the prior art. The service side is joined with branches and the high-speed line side with high-speed channel. At the transmitting direction, after the received optical signals are converted to electrical signals by a receiving module 201 in OTU, a Clock & Data Recovery module (CDR) 202 extracts clock frequency information from the service data. After converting the electrical signals processed by CDR module 202 to optical signals complying with standards and proper for DWDM system transmission, a transmitting module 203 at the service side sends out the service data utilizing the extracted clock frequency. In like manner, at the receiving direction, after receiving the signals and converting optical signals to electrical signals, a receiving module 204 at the high-speed line side transmits the signals to CDR module 205, the received service data are transmitted to transmitting module 206 at the high-speed line side by CDR module 205 according to the clock frequency of this service data, then sent to corresponding ports after the electrical-optical conversion process by transmitting module 206 at the high-speed line side.
A CDR module can implement data recovery and clock frequency extraction of data services with arbitrary speed, which avoids designing wavelength conversion board with different OTUs for accessing different services in the same system, so that the system is provided with better compatibility. For instance, a CDR module can be designed to support clock frequency extraction of data services with arbitrary speed ranging 10M□2.7 Gbps. In this way, one OTU can receive or transmit data service with different speed in order to meet requirement of different speed services.
However, this method has the following disadvantages: when a low-speed service is accessed to an OTU, for example 125 Mbps Fast Ethernet (FE), one 125 Mbps will occupy one wavelength resource, so that waveband resource of wavelength is severely wasted. Meanwhile, as the transmitting/receiving modules at the high-speed line side are special modules according with standard wavelength, the cost is very high when using these special modules to transmit low-speed services.
Another transfer mode is to converge the multi-port services. To solve the problem of bandwidth utilization ratio, a general method is to converge several low-speed services to one high-speed channel and then transmit them.
FIG. 3 is a schematic diagram of OTU for implementing multi-port service convergence according to the prior art. Every one port corresponds to a branch, and different branches are used to transmit service data with different speeds, but one branch is only used to transmit service data with constant speed. As for uplink transmitting direction, namely when OTU is transmitting information at service side, taking branch 1 for example, firstly the received optical signals are converted to electrical signals and then transmitted to CDR module 302a for receiving constant speed service by optical-electrical converting module 301a; CDR module 302a for receiving constant speed service extracts clock frequency information of service data from the received service data, then obtains pure service data by removing filling characters or overhead characters in the service data; the pure service data is sent to encapsulation module 304 after being processed by serial-parallel converting module 303a. The encapsulation module 304 encapsulates service data from every port in terms of the pre-defined encapsulation format, which usually contains frame header, frame trailer and channel information convenient for correctly restoring the service data in downlink direction, such regulations as High-level Data Link Control (HDLC), Link Access Procedure-SDH (LAPS), General Frame encapsulation Process (GFP) or other encapsulation formats approved by OTU can be adopted for encapsulation. Then mapping module 305 maps the encapsulated service data to the container of high-speed channel; after parallel-serial converting process and electrical-optical converting process by parallel-serial and electrical-optical converting module 306, the service data is sent out in terms of the high-speed channel clock frequency in high-speed channel transmitting clock module 307. For example, the encapsulated service data can be mapped to the container of a certain transfer format under SDH format, such as container of STM-16 VC3/VC4. In the course of mapping, if bandwidth of the received service is greater than container capacity, multiple containers are lashed together for use; if bandwidth of the received service is smaller than container capacity, filling characters or gap packets are inserted to format of encapsulation definition in order to implement bandwidth adapting.
In the same way, as for downlink transmitting direction, namely when the high-speed line side of the OTU has received information from high-speed channel, firstly the received optical signals are performed with processes of optical-electrical converting, line clock frequency extraction and serial-parallel converting by converting module 308, in order to guarantee that the clock frequency for transmitting data in downlink direction is consistent with that of high-speed channel; then de-mapping module 309 performs de-mapping, namely restoring service data of each branch from high-speed channel container and discarding potential filling characters and gap packets in mapping course at the same time; then decapsulating module 310 performs decapsulation in accordance with encapsulation format, in order to restore the specific service data of each branch. Since in uplink transmitting course, the process such as encapsulating and mapping is not performed to all service data of each branch, but to the pure service data extracted from service data, the clock frequency information of each branch has been totally lost. And in downlink transmitting course, clock frequency of each branch should be the same as that of uplink transmitting course. However, as the clock frequency information of each uplink branch has been totally lost, it is impossible to restore clock frequency information of uplink transmitting course in downlink transmitting course. Under this condition, speed adapting must be performed in decapsulating module 310. Concretely speaking, according to the requirement of different protocols, some gap packets according with regulation of the transmitted service are inserted by speed adapting modules in decapsulating module 310, in order to eliminate frequency deviation. For instance, as to Fibre Channel Protocol service, some idle signals of Fibre Channel can be inserted; as to Synchronous Digital Hierarchy/Synchronous Optical Network (SDH/SONET) service, bit adjusting can be performed. After the service data adjusted by speed adapting module is processed by parallel-serial converting module 312a and electrical-optical converting module 313a, namely after the process of parallel-serial converting and electrical-optical converting, the service data are sent out according to the frequency generated by branch clock generating module 311a of the present branch.
This method also has one disadvantage: since the clock frequency of branch corresponding to each OTU port cannot be transparently transmitted, frequency deviation between transmitting side and receiving side is unable to be recognized. In OTU, the receiving side must perform speed adapting in order to eliminate frequency deviation. However, speed-adapting operation is associated with service types, so it is very difficult to design an OTU which is suitable for an arbitrary speed. Therefore, several common module types are designed, for example: 2×GE is converged to STM-16, 10×ESCON is converged to STM-16, and 4×STM-4 is converged to STM-16. But it proves difficult to make various modules compatible. In this way, the flexibility of OTU is low and it is difficult to implement compounding service transmission. For instance, to implement convergence from 1×GE+1×STM-4 to STM-16 is quite difficult. In order to adapt to various kinds of service convergence, various kinds of OTU modules must be designed to adapt speeds of various kinds of services, which make the system design cost increase. In conclusion, convergence of services with arbitrary speed cannot be implemented with this method, resulting in low flexibility and high cost.